This invention relates to the measurement of thickness of sheet material, particularly sheet material as it moves through a rolling mill, and control of the rolling mill.
In the past, two approaches have been utilized in the measurement of sheet thickness. In lower speed mills, typically where the metal strip moves at a speed of 2,000 feet per minute and lower, mechanical thickness gauges are used. The mechanical gauges provide accurate and direct measurement of sheet thickness so long as uniform contact is maintained with the sheet. However as the speed increases, the feelers begin to bounce and ride off the sheet resulting in a practical upper limit of sheet speed for mechanical type thickness gauges.
Radiation type thickness gauges are used with the higher speed rolling mills, some of which operate in the range of 4,000 to 8,000 feet per minute. A conventional radiation thickness gauge utilizes a radiation source on one side of the sheet and a radiation detector on the opposite side. The radiation detector measures the radiation transmitted through the sheet in terms of counts, with the number of counts being a function of the mass absorption coefficient, the density and the thickness of the sheet. More specifically, the radiation at the detector is proportional to e-.mu..rho..sup.t. Mu and Rho vary with the composition of the material, varying both with the specific component and the amount of the component present in the material. The conventional radiation thickness gauges consider the product of Mu and Rho to be a constant and a nominal value for this constant is provided for each alloy. For example, aluminum 2024 will have a first value for the constant, aluminum 3004 will have a second value, and aluminum 5082 will have a third value. When aluminum 2024 is being run through the rolling mill, the calibration knob of the thickness gauge is set to the calibration figure provided for this particular alloy. The calibration figure may be taken from a chart which provides the value for the nominal composition of the alloy. However this is not satisfactory for many situations since the amounts of the various constituents in the alloy will vary. Greater accuracy is achieved by taking a sample of the material and making a spectographic analysis in the laboratory providing a calibration figure for the sample. However this procedure has not been satisfactory because of large variations from ingot to ingot even though cast together, and from roll to roll even when rolled from the same ingot, as well as variations in composition within a roll. Errors as high as 15% in determining the Mu Rho product for a sample have been encountered.
Ordinarily a radiation thickness gauge utilizes an X-ray or gamma ray source such as an X-ray tube or a radioactive isotope. Attempts to reduce the error in gauging have led to the use of beta radiation sources for thickness gauges. However while the errors are reduced, the beta sources require positioning the source and detector very close to the moving sheet which is very undesirable.
It is an object of the present application to provide a new and improved radiation type thickness measuring apparatus and method particularly suited for high speed rolling mills which substantially reduces the errors of prior art systems while being operable with X-ray and gamma ray sources with relative wide spacing from the sheet being measured.